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I am still not there on this one.
I modified the PI to account for the fact that now, my signals mix into only one side of the PI. After reviewing the output signals on a scope, I decided to change my plate resistors to 100k/110k. I wanted to try 82k/100k but did not have the parts on hand.
With this setup and a JJ long plate in the PI tube slot, I found that my signal was reasonably balanced. Close enough for me (for now...) but I still need to experiment with different tubes for the PI position and may thus continue to tweak the plate values.
I also lowered the bias resisor of the PI from 1.2k to 680. My goal was to compensate for losses due to mixing/running effects in the loop. I am unsure where I will eventually settle in this regard, and more playing time is requred before I commit to a specific value. I left the tail alone for now, but may mess with that at a later time as well.
Also, I measured the input impedance of the PI and noted that it was approxiately 2meg. As such, I decided to lower the value of the PI coupling cap. I am shooting for tight, yet full bass. I figured that eliminating some low frequency information might tighten up the tone. Unfortunately, I did not have a lot of extra caps on hand, but I did sub in a 0.003uf cap (IIRC), which puts my knee right around 24hz and this worked out pretty well, and this should add an extra layer of filtering, depending upon the effects that I use in the new loop.
I did not have a lot of time to fully evaluate these changes, and will do so in greater detail this week. However, I did find that the overall volume of my modified TC-15 was comperable to or exceeds other 15-18 watt EL84 amps that I have on-hand to compare to. Also, the changes are preliminarily having a minimal impact on the tone of the amp as originally configured, at least as far as my ears "remember", with the notable exception that now I have a loop for reverb/delay etc. and an extra stage to drive the EF86 harder.
There is a ton of good reading scattered across the net. Enough for anyone to figure out how to mod this style of amp to their very own personalized tastes. That said, these information resources are best read/studied with an open chassis, an oscilloscope and an open mind all put together in the context of your own needs.
For example, please keep in mind that these tweaks that I have been working on are based upon my particular guitars (tele and strats) with relatively weak output pickups and are further based upon my playing style (country) and are based upon the existing equipment/setup/speakers that I plan on using the amp with. I basically tweak, then listen/play repeat... Your milage may thus vary.
Also, I am interested in hearing other reports of mods to these amps. For example, I have been kicking around the idea of splitting the cathodes of V1 of the TB side to use one triode as a mixer instead of the mixing scheme described above. So far, the current approach is working, so I may try this as a purely academic excercise.
As a side note, the relays have been shipped, but I have not received them yet, so I have not done a lot with the chanel switching on this amp yet, other than to note that my manual switches are working out fine.
Again, I am hesitant to provide a revised schematic/layout until I am happy with the changes and have convinced myself that I will be keeping the revised values. Until then, I hope my explanations are sufficient to steer you all as to the directions I am exploring.
Best regards
Tom
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